1. Field of the Invention
The present invention relates to an auto-focusing device used in photographing or video imaging, and more particularly to an auto-focusing device of a camera which uses an exchangeable lens.
2. Related Background Art
Today, a camera having an auto-focusing (AF) function is a primary one. In such an AF camera, an image is stored in an AF sensor, a correlation calculation is done based or the stored sensor output signal to calculate a defocus amount to an in-focus position, and if the defocus amount is not in the in-focus state, a lens drive amount is calculated from a relational formula of the defocus amount and the drive amount of the focusing lens, and the lens is driven into the in-focus state.
A sensitivity of the lens is a coefficient to be used in the calculation of the lens drive amount based on the defocus amount and it is determined by a focal length of the lens and a characteristic of a drive control unit. There is a following relation between the defocus amount and the sensitivity. ##EQU1## where V : lens drive amount
d : defocus amount PA1 S.sub.0 : 0-order sensitivity PA1 S.sub.1 : 1-order sensitivity PA1 S.sub.2 : 2-order sensitivity PA1 S.sub.3 : 3-order sensitivity
Where a lens system mounted on the camera is a zoom lens, the sensitivity varies with the zoom position. Accordingly, the sensitivity is read when the drive distance is to be calculated.
In a recent focusing system of the lens system, an inner focus system in which a driven lens system is small and light in weight is frequently used. In the inner focus system, because the lenses used for focusing are small and light as described above, a load to a driving actuator is small and the actuator itself may be small and rapid drive may be attained. It also has an advantage over a front lens drive system in that a minimum photographing distance is shorter. However, in the inner focus type lens system, since the sensitivity greatly changes depending on the position of the focusing lens, if the AF operation is done while the focusing lens system is driven (overlapped operation), the sensitivity read in the calculation of the lens drive distance and the sensitivity at the time of storing in the sensor differ from each other because the lens position at the time of storing in the sensor and the lens position for use in the calculation of the lens drive amount in the AF calculation are different from each other, and the calculated lens drive amount is different from an actual lens drive amount.
The above drawback is explained with reference to FIG. 3. FIG. 3 shows a graph of change of sensitivity by the position (position of a distance ring) of the focusing lens for a telescope end of a zoom lens having a focal distance of 35-135 mm. A broken line is for a front lens drive type lens, and a solid line is for an inner focus type lens. As seen from FIG. 3, the inner focus type lens shows a larger variation in the sensitivity than that of the front lens drive type (In FIG. 3, they are normalized so that the sensitivity at an infinite position is 1.0). For example, when the distance ring is driven from 2 m to 1.3 m during the accumulation in the sensor, the sensitivity changes from 0.95 to 0.93 for the front lens type (broken line) but it largely changes from 0.62 to 0.5 for the inner focus type (solid line). When the defocusing distance has been calculated and the sensitivity is read, the sensitivity would have further changed and it would be significantly different from an average sensitivity during the accumulation in the sensor.
Thus, the change is negligible in the front lens type but the change is too large in the inner focus type to correctly calculate the lens drive distance.